1. Field of the Invention
The present invention relates generally to electrophotographic image-forming members or photoreceptors comprising hydrogenated amorphous silicon (a-Si:H) formed onto a supporting conductive substrate. More particularly, this invention is directed to an improved a-Si:H photoreceptor and to a method for making the photoreceptor.
2. Discussion of the Prior Art
Electrophotography is a well-known image transduction art depending on the formation of an electrostatic latent image on a charge-sensitized photoconductor formed onto a suitable substrate. The latent image is typically produced by photo-induced discharge of the photoconductor in response to a light image projected onto the working surface of the photoconductor, and a visual image for transfer to the hard-copy medium is developed from the latent image by contacting it with charge-sensitive toner particles. The toned image is then the basis for a variety of further imaging processes. The versatility of electrophotography has permitted its application in systems for copying, duplicating, printing, plate making and color proofing, among others, and electrophotography is increasingly being applied in computer output devices in which lasers are used to produce the latent image. Commercial potential of such systems is directly affected by the performance and producibility of the photoconductor. Generally, the photoconductor must have good charge acceptance V.sub.0 and a long dark decay .tau..sub.D, typically 10 to 20 seconds at minimum. In addition, fast photo-induced discharge is required, and the spectral response of the photoconductor must be compatible with the source. In critical applications, photoconductor fatigue or residual voltage may be limiting.
Considerable effort has been expended in development of prior-art photoconductors based on inorganic materials such as cadmium sulfide, zinc oxide, or selenium, as well as organic materials such as TNF-PVCz (the reaction product of 2,4,7-trinitro-9-fluorenone and poly-9-vinylcarbazole). Some prior-art photoconductors suffer well-known disadvantages such as low charge acceptance or short dark decays, poor thermal or environmental stability, poor mechanical strength or durability, or the potential for environmental contamination. Further, others lack good adhesion properties or are otherwise incompatible with use of flexible substrates required by large-format applications such as color proofing. In addition, many require formation temperatures too high to permit their use with plastic substrates.
High-quality, large-format electrophotography can be practiced through use of microcrystalline cadmium sulfide deposited onto thin conductive substrates (U.S. Pat. Nos. 4,025,339 and 4,269,919). A metallic member, or a plastic member coated with a metallic or an ohmic layer, may form such conductive substrate. Sputtered to thicknesses of 0.3 to 5 micra onto stainless-steel roll-stock up to one meter wide and about 0.1 mm thick, such anisotropic photoconductors have been adapted to provide flexible photoreceptors for an analog color-proofing application (U.S. Pat. Nos. 4,358,195 and 4,556,309). This application required that a latent image be retained almost two minutes between photoreceptor charging and development. The large-format (approximately 50 cm by 60 cm) photoreceptors were required to be reusable for thousands of operational cycles. During operation, at 105 seconds after corona charging these thin cadmium sulfide photoconductors demonstrate typical surface potentials V.sub.105 =22 volts, they have linear photo-induced discharge, and they yield substantially zero residual voltage on complete discharge. When used with optimized liquid toner systems, these flexible low-voltage photoreceptors provide high-resolution four-color proofs. However, a potential environmental hazard due to manufacture or disposal of the cadmium sulfide photoconductor remains a concern.
The disadvantages of prior-art photoconductors has prompted investigation of amorphous silicon (a-Si) as the photosensitive material for use in electrophotographic photoreceptors. Amorphous silicon poses no environmental hazard and has good mechanical strength, adhesion, and durability, but demonstrates undesirable characteristics thought to originate in unsatisfied (or dangling) bonds in the silicon matrix. It has been shown that formation of amorphous silicon in presence of hydrogen provides a material (a-Si:H) with fewer dangling bonds and improved characteristics, the greatest improvement occurring for deposition substrate temperatures of approximately 230.degree. C.
An extensive art based on a-Si:H materials has developed in the field of solar-energy conversion, in which thin a-Si:H layers are routinely deposited onto large-area flexible substrates; the internal resistance of such photovoltaic devices must be as low as possible (of the order of 100 ohms) for attractive power outputs, but the corresponding volume resistivities (about 10.sup.6 ohm.multidot.cm) result in photoconductive properties ill-suited to electrophotographic applications. Other a-Si:H materials made to have higher resistivities exhibit attractive photoconductive properties, and by appropriate doping, such a-Si:H photoconductors can be made to accept positive charging, negative charging, or charging in either polarity. However, conventional a-Si:H photoreceptors are typically directed toward rapid-imaging systems for office use, the toner systems for which may require surface potentials of 100 volts or greater but the operational cycles for which seldom require dark decays longer than a few seconds. Consequently, the prior-art a-Si:H photoreceptors (e.g., U.S. Pat. No. 4,265,991) have demonstrated several characteristics which limit their usefulness as low-voltage electrophotographic photoreceptors. Included are the following significant disadvantages:
1. The low dark volume-resistivity (about 10.sup.10 ohm.multidot.cm) of such a-Si:H photoconductors, and their resultant fast dark decays, require deposition of a high-voltage a-Si:H layer at least 10 (and usually 20 to 50) micra in thickness to achieve the surface potentials needed by many electrophotographic processes at toning; these thick photoconductive layers are both expensive to produce and poorly adapted to use with flexible substrates. As is known in the art, long dark decays require that a photoconductor have both a wide optical bandgap, which indicates a low density of thermally generated charge carriers, and a low drift mobility for such carriers. The optical bandgap of a-Si:H is known to increase with increasing hydrogen content, up to about 10% total hydrogen, and carrier mobilities in a-Si:H are known to decrease with addition of small amounts of neutral dopants such as oxygen or nitrogen. However, prior-art a-Si:H photoconductors based on either bandgap widening by hydrogen enrichment or mobility suppression by doping-induced trapping enhancement demonstrate degraded photoconductive properties and spatial inhomogeneities in the charge acceptance or toning response. In addition, the bulk properties of prior-art a-Si:H photoconductors are adversely affected by interface processes. When prior-art a-Si:H photoconductors are used in bilayer photoreceptors, carrier injection from the conductive substrate or charge transfer from the environment accelerates bulk dark-decay processes, further reducing applicability of such photoreceptors. Such processes have been partially overcome in the prior art by fabrication of multilayered photoreceptors in which thin (a few hundred nm or less) insulating layers are either deposited at the interface between the a-Si:H photoconductor and the conductive substrate, or topcoated over the photoconductor, or both. PA1 2. Charge injection or impurity migration into the adherent surface of the photoconductor has been particularly limiting for a-Si:H photoconductors formed onto many conductive substrates. In the prior art, thin blocking or barrier layers are commonly deposited on the substrate surface prior to formation of the a-Si:H photoconductor; both electrically insulating and less-reactive conductive materials have been used. Yet another approach has been to use the a-Si:H photoconductor as a charge-generation layer and couple it with another layer which acts as a charge-transport layer. These multilayered photoreceptors are complicated to process, costly to produce, and still require a-Si:H layers at least 10 (and typically 20 to 50) micra thick to achieve practical surface potentials at toning; in addition they are inflexible and difficult to manufacture in the large formats required for electrophotographic applications such as analog or digital color proofing. PA1 3. Electrophotographic properties of a-Si:H photoconductors degrade on exposure to environmental humidity or to reactive species present during charging. This sensitivity is thought to originate in unsatisfied dangling bonds on the surface of the a-Si:H photoconductor, as well as on surfaces of internal structural inhomogeneities accessible to active species. In the prior art, such sensitivity has been decreased by overcoating the a-Si:H photoconductor with a thin (of the order of 10 to 200 nm thick) electrically insulating topcoat such as silicon nitride, silicon carbide, or silicon dioxide; however, such prior-art topcoatings add cost, typically require a separate deposition step, and may give rise to an undesirable residual voltage unless kept ineffectually thin. PA1 1. Both the density and refractive index of such a-Si:H photoconductors are notably less than that for crystalline silicon, both decreasing as the substrate temperature at deposition is decreased. It is widely accepted that these effects are due to both formation of microvoids in the photoconductor and segregation of electronically detrimental non-monohydrides and contaminants on the surfaces of such voids. PA1 2. Bonding of hydrogen in non-monohydride modes is favored, with probable concentration in the voids and attendant instability in photoconductor properties. PA1 3. Prior-art a-Si:H photoreceptors formed at substrate temperatures of less than about half of silicon's melting temperature are known to exhibit scanning electron microscope (SEM)-resolvable columnar growth structure 10 to 100 nm in diameter; the columns are separated by interstices due to incomplete coalescence of nucleation islands. Columnar interstices originate at the substrate and propagate through the photoconductor thickness. They not only decrease photoconductor density and refractive index, but also act as segregation surfaces, and serve as diffusion channels for reactive species, so degrading photoconductor properties through increased environmental sensitivity. Such interstitial effects can dominate bulk properties of the columns, particularly in thin layers, and are widely thought to account for the poor electrophotographic performance of prior-art photoreceptors incorporating a-Si:H photoconductors. These effects are particularly limiting if the photoreceptor comprises a thin a-Si:H layer deposited onto a conductive substrate.
These disadvantages typify prior-art a-Si:H photoconductors prepared by either silane-based glow discharge or reactive sputtering based on admixed hydrogen in the sputtering atmosphere. Both preparative methods have been used to deposit prior-art a-Si:H photoconductors onto metal substrates, usually at substrate temperatures of 250.degree. C. or greater. Glow-discharge methods have yielded the best prior-art a-Si:H photoconductors, but deposition rates are low and even further limited if dark resistivities of the order of 10.sup.10 ohm.multidot.cm are to be obtained. In addition, damaging reactions between process gases and plastic substrates preclude deposition of a-Si:H photoconductors onto such substrates by glow discharge methods.
Prior-art methods for making a-Si:H photoreceptors yield poor results when used at low substrate temperatures. As has been noted, the prior-art a-Si:H photoconductors having the best properties were made by silane-based glow-discharge processes; these deposit SiH.sub.x species onto the substrate, with bonding of the Si atoms and diffusion and evolution of excess hydrogen. Under optimum conditions, a-Si:H layers with low non-monohydride content results, but at low substrate temperatures polysilane layers are likely to result. In contrast, photoconductive layers made by sputtering processes are built up by deposition and fusion of Si atoms removed from the target, simultaneously with hydrogenation due to the sputtering atmosphere. Under prior-art sputtering conditions, such processes tend to produce SEM-resolvable columnar microstructure, which at low substrate temperatures results in low-density layers of particularly poor photoconductive qualities.
Sputtering methods can achieve deposition rates several times greater than are currently available by glow-discharge processes and thus promise greater commercial utility. Unfortunately, the a-Si:H photoreceptors prepared by prior-art radio-frequency (RF) reactive-sputtering practices suffer especially from the above-listed disadvantages, and it is known that a-Si:H photoreceptors sputter-deposited at substrate temperatures below about 250.degree. C. have particularly poor properties and stability. At such substrate temperatures, sputtering conditions favor several mechanisms considered to be deleterious to a-Si:H photoreceptor performance:
It is known that substrate potential during a-Si:H deposition can significantly influence both the structural and compositional homogeneity of the a-Si:H layer being deposited. It is also known that the greater mobility of electrons in the plasma causes conductive substrates to spontaneously develop an uncontrolled negative potential (self-bias) relative to the plasma and apparatus structure that is dependent on the deposition conditions. In RF sputtering apparatus, the magnitude of the self-bias so developed depends most directly on the partial pressure of argon (P.sub.Ar) in the sputtering atmosphere, on the ratio of the effective areas of substrate and target, and on the target voltage V.sub.T established by the diode action on applied RF power. Substrate self-bias is commonly controlled by varying target voltage V.sub.T, which also affects the a-Si:H deposition rate R.sub.Si. For prior-art sputtering conditions, maximum substrate self-bias potentials are approximately -20 volts, and reduction of target voltage to vary the self-bias potential reduces R.sub.Si. Thus, control of substrate self-bias by varying target voltage V.sub.T is counterproductive when high deposition rates are desired.
An external positive bias potential has been applied to the substrate (relative to the apparatus structure) to minimize bombardment of the developing a-Si:H layer by Ar.sup.+ and Si.sup.+ ions, but this approach results in increased electron bombardment, heating of the developing a-Si:H photoconductive layer, and probable damage to plastic substrate materials. An external negative bias potential has been applied to the substrate, to limit electron bombardment and consequent heating of the a-Si:H layer; however, this results in hydrogen etching of the developing a-Si:H layer, in back sputtering of the a-Si:H layer by Ar.sup.+ ions, or in increased incorporation of argon into the a-Si:H layer, all mechanisms considered in the prior art to result in undesirable structural and compositional inhomogeneity through said layer. Such prior-art external bias potentials are typically applied without apparent consideration of the level of substrate self-bias and are limited to a magnitude of approximately 20 volts measured at the voltage source. Any beneficial effect of bias current on photoconductor properties is unrecognized in the prior art. Photoreceptors comprising a-Si:H layers sputtered under prior-art bias conditions demonstrate dark decays too rapid for use in many electrophotographic applications and particularly so for those requiring thin photoconductive layers.
Because of the poor electrophotographic properties demonstrated by prior-art a-Si:H photoreceptors, and particularly for those sputtered at substrate temperatures below 200.degree. C., there is a need for an improved a-Si:H photoconductor depositable by reactive sputtering methods. Moreover, prior-art sputtering methods preclude use of plastic substrates, which offer advantages in many electrophotographic applications but which may deform unacceptably at substrate temperatures greater than approximately 130.degree. C. In general, photoconductor deposition at substrate temperatures of approximately 130.degree. C. would result in significantly fewer thermal defects in the photoconductor, in decreased cooling stress between the photoconductor and its substrate, and in significantly lower process energy costs, all of which are advantageous for commercial production of a-Si:H photoreceptors.
Thus, it is desirable to provide an improved a-Si:H photoconductor which, in thicknesses of five micra or less, retains sufficient surface potential to permit effective toning of its latent image in practical electrophotographic processes; minimum dark decays of 20 seconds are required, while dark decays of two minutes or more are needed in many applications, particularly in digital ones using lasers to write large-format optical images. It is desirable that the method provide deposition of such a-Si:H photoconductors at significantly lower substrate temperatures than are used for prior-art a-Si:H photoconductors, so that plastic substrates may be used. It is desirable that the photoconductor exhibit minimal growth in the columnar habit, so that interstitial segregation and diffusion effects on its electrophotographic properties and their stability can be limited. It is further desirable that the photoconductor be directly depositable onto flexible conductive substrates, to which it should be firmly adherent.
It is also desirable to provide an improved method for forming the improved a-Si:H photoconductor and incorporating it into electrophotographic photoreceptors. It is desirable that such method be capable of depositing photoconductors of uniform spatial properties onto large-format flexible substrates. It is desirable that no blocking or barrier layers be required between the photoconductor and the conductive substrate. It is desirable that the forming method be practicable at significantly lower substrate temperatures than are prior-art sputtering methods, so that plastic substrates may be used. It is desirable that critical process parameters of the forming method be amenable to control. It is desirable that the forming method permit doping of the a-Si:H photoconductor, as well as deposition of an insulating topcoat onto it, if desired.
To address the first two above-listed disadvantages of prior-art a-Si:H photoconductors, a novel photoconductor and method have been developed. To address the third of the above-listed disadvantages, a cost effective method for stabilizing properties of a-Si:H photoreceptors in high-humidity environments is disclosed in the above cross-referenced related application, which is herein incorporated by reference. Said stabilization method is usable with the present invention and comprises a surface treatment of the photoconductor with a silanol-rich prepolymer, a rinse with water, and a heat treatment as therein described.